Induction motor



1949 I E. w. BALLENTINE 2,492,207

INDUCTION MOTOR Filed Nov. 22, 1946 2 Sheets-Sheet 1 Dec. 27, 1949 FiledNOV. 22, 1946 E. w. BALLENTINE 2,492,207

INDUCTION MOTOR 2 Sheets-Sheet 2 ssool 7 3400- 3200- 2600- 2' 2400- Q.2200- 05 2000- 31600- Q 1600- 1400- Lu Q 1200- D 1000- TORQUE v I N VENTQR.

Eg ie lflfiaienine.

Patented Dec. 27, 1949 INDUCTION MOTOlt Earle W. Ballentine, Chicago,111., assignor to Russell Electric Company,

poration of Illinois Chicago, 111., a cor- Application November 2 2,1946, Serial No. 711,694

5 Claims.

The present invention relates to induction motors and more particularlyto fractional horsepower motors of the shaded-pole type. Specifically,the present invention is an improvement on the invention disclosed andclaimed in my prior copending application for Letters Patent of theUnited States, Serial No. 608,419, filed August 2, 1945, Patent No.2,454,589, issued November 23, 1948, and assigned to the same assigneeas the present application.

The shaded-pole motor is in a great many applications at the presenttime. It is a well known fact that the shading coils used in connectionwith a shaded-pole motor for linking a portion of the flux in the polecause a lag in the flux which passes through the shading coils relativeto the flux not linked by the shading coils so that there is in ashaded-pole motor energized from a single phase source a tendencytowards the rotating field so well known in connection with polyphasemotors for example. The theory underlying the operation of shaded-polemotors is not too well understood. However, the shaded-pole motors ofthe prior art have been notoriously low in efficiency and in addition,the characteristic speed-torque curve included a decided dip in theneighborhood of 1600 R. P. M. for a two-pole motor. Also the used veryextensively greatly increased efiiciency and torque relative to similarshaded-pole motors used heretofore while employing a construction thatis no more expensive either from the standpoint of labor or materialsthan such motors used heretofore.

It is a feature of the present invention to provide a two-poleshaded-pole induction motor emplaying in addition to two shadingwindings per pole a reluctance ga-p extending over a portion of eachpole face so that effectively a four-phase motor is provided, the phasesbeing characterized by the instantaneous space positions of the airgapflux along each pole face with the most leading flux being thatassociated with the reluctance gap, followed by the main flux, the fluxlinked by only one shading coil, and the flux linked by two shadingcoils in that order.

It is a further object of the present invention to provide an effectivefour-phase, two-pole shaded-pole induction motor having greatly improvedcharacteristics.

Further objects and advantages of the present invention will becomeapparent as the followstarting torque has been low so that such priorart motors were unsatisfactory for many applications. In the motordisclosed and claimed in my prior copending application, it was foundthat by employing a relatively non-homogeneous arrangement of stator androtor elements that the efliciency of such a shaded-pole motor wasconsiderably increased and ,the starting torque, for example, wasincreased by as much as 35 percent. It would 'be desirable to provide ashaded-pole motor costing no more to manufacture than the shaded-polemotor described in my above-mentioned copending application which notonly has a greatly increased eiliciency and starting torque but inaddition, is characterized by the absence of a dip in the speed-torquecurve.

Accordingly, it is an object of the present invention to provide a newand improved shadedpole motor having increased starting torque andefllciency.

It is another object of the present invention to provide a shaded-polemotor in which the speed-torque curve is characterized by the absence ofany dip.

It is a further object of the present invention to provide a shaded-poleinduction motor having ing description proceeds and the features ofnovelty which characterize this invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

For a better understanding of the present invention, reference may behad to the accompanying dra 'ngs in which:

Fig. 1 is a perspective view of a core type shaded-pole motor embodyingthe present invention;

Fig. 2 is an enlarged view of a portion of the stator and rotor of themotor of Fig. 1 to more clearly bring out the invention;

Fig. 3 is a sectional view taken on line 3-3 of Fig. 2 with the rotorremoved;

Fig. 4 illustrates for comparison purposes speedtorque curves for themotor of the present invention and the best of similar types ofshadedpole motors of the prior art; and

Fig. 5 is a view similar to Fig. 2 illustrating a modification of thepresent invention.

In practicing the present invention, the elementsof the motor except forthe stator laminations may be identical with the elements of the motordisclosed and claimed in my prior copending application referred toabove. Instead, however, of the airgap flux per pole having threedifferent phase characteristics, a four-phase flux is produced byproviding in addition to the main alrgap flux and the two sets of fluxesembraced by only one and by more than one shading coil,

' a leading flux obtained by providing over a portion of the pole facedrawings, generally of lami- From the transformer analogy, the socalled"core type stator I is one where the energizing winding encloses theentire section of shell-type stator. As shown in Fig. 1 of the drawings,the stator l0 actually like It which embraces the For a particular arectangular core having outside dimensions of 2 /4 by 2% I31: by theends of the spool H is As in the core type motor of my prior copendingapplication referred to above, the stator lamtwo-pole induction motor ofthe present invention. As in my prior copending application, the tips ofthe poles 20 and 2| extend around the rotor as indicated at 20a and Zlato form what are generally referred to as integral magnetic bridgeswhereby some flux from each pole 4 may be diverted to the other polewithout passing through the airgap between the rotor II and the statorIll. The purpose of these integral is to increase prior application andas illustrated in the drawings, is of the prising an iron core formed oflaminations similar to the stator laminations assembled in a stackhaving an over-all thickness equal to the thicksuch as 30 in the statorlaminations I I. 11' desired, a suitable disk 3| of non-magneticmaterial having upset tan-like projections 32 may be provided at one orboth ends of various ones 01 the blades If desired the be verysatisfactory. An airgap 01' 15 mils has desirable for the specificdesign of notor mentioned above. It should be understood, however, thatthe suggested use of an even number of rotor bars is by way of exampleonly since particularly for high torque applications an odd number ofrotor bars may be very satisfactorily employed.

As was described and claimed in my prior copending application referredto above, the stator I8 is provided with a plurality of shading coils orshading rings for each pole, two being shown for each pole in Fig. 2 ofthe drawings. The shading coils or windings for the pole 20 aredesignated by the reference numerals 35 and 36, while the shading coilsfor the pole 2| are designated by the reference numerals 31 and 38. Theshading coils 35 and 31 effectively link the flux passing through asubstantially greater area of the pole face than the shading coils 36and 38 as is clearly apparent from the drawings. The shading coils 36and 38 embrace only a portion of the pole structure which is alreadyembraced by the shading coils 35 and 31 respectively. These shadingcoils are positioned in suitable slots or tunnels formed in the statorlaminations closely adjacent the airgap between the rotor l8 and thestator l0. As illustrated, the portion of the shading coil 35 adjacentthe rotor I8 is positioned in the slot or tunnel 39. The portion of theshading coil 36 adjacent the rotor 18 is positioned in the slot ortunnel 40. Similarly, the portion of the shading coil 31 adjacent therotor I8 is positioned in the slot or tunnel 4| and the portion of theshading coil 38 adjacent the rotor I8 is positioned in the slot ortunnel 42. Suitable notches, all designated by the reference numeral 43are provided in the stator laminations II to accommodate one other sideof the shading coils or rings 35, 36, 31 and 38 which preferably areformed of one turn of a very low resistance conductor. It will beunderstood by those skilled in the art that a shading coil has itsmaximum effect in retarding the flux when it has a minimum resistanceand a maximum inductance. As illustrated, the shading coils 35, 36, 31and 38 comprise a heavy copper wire or the like. slots 39, 40, 4| and 42are in accordance with my prior copending application referred to above,positioned in a non-symmetrical manner relative to the symmetricallydisposed rotor bars 22 whereby as the rotor 18 rotates relative to thestator III, the particular rotor bars adjacent the tunnels or slots 39to 42, inclusive, come into alignment successively. In other words, atany instant if a particular rotor bar 22 is in alignment with aparticular slot or tunnel such as 39 for example, then the rotor bars 22adjacent the other slots or tunnels 40, 4| and 42 are not in alignmentat that instant and in fact, are out of alignment by different varyingamounts, thereby producing a much more satisfactory operation than inprior art arrangements. It will be noted that the shading rings orwindings 35 and 36 which are each effectively a one-turn coil areangularly displaced with respect to each other as is also the caseinconnection with the shading rings 31 and 38. This is more fullydescribed in my prior copending application referred to above and formsno part of the present invention. It should furthermore be understoodthat these shading coils could be disposed in generally parallel planesif desired and although only two shading coils per pole are illustrateda plurality per pole such as three or more per pole might be employed.Three shading coils per pole would The tunnels or lit for thearrangement of Fig. 1 provide an effective five-phase motor.

For the purpose of bringing out generally the distribution of the fluxin the airgap between the rotor 18 and the stator I'O each pole such asthe pole 20 may be considered as comprising the sections or teeth 44,45, 48 and 41, respectively. The tooth 44 may be considered to be thatportion of the pole 26 between the tunnel 48 and the center line l3,while the tooth 45 may be considered to be the portion of the pole 28between the tunnels 39 and 40. In my prior copending application theremainder of the pole 28 might have been considered to be the tooth 46.In accordance with the present invention, however, the tooth 45 onlyextends between the slot or tunnel 39 and the point 46a and the portionof the pole 20 between the point 46a and the point 41a is designated asthe tooth 41. There is desi nated on pole 20 in Fig. 2 of the drawingsadjacent the teeth 44 to 41, inclusive, the symbols ps bsi, (Pm and r,respectively designating the magnetic fluxes varying in phase relativeto one another and disposed in the airgap between the pole 20 and therotor 18. The symbol 4m refers to the flux passing through the tooth 44of the pole 20 linked by both of the shading coils or windings 35 and36. The symbol s1 designates the flux passing through the stator tooth45 linked by only the shading coil or winding 35 of the pole 2|. Thesymbol m designates the flux passing through the main pole tooth 46. Aswill be brought out hereinafter, the symbol r designates the fluxpassing through the stator tooth 41. It will be understood by thoseskilled in the art, the flux s1 is lagging in phase relative to the fluxm by a predetermined amount and the flux sz lags by a still greateramount. Considering for the moment only the fluxes dim, s1, and sz, itis apparent that with this arrangement there is produced in effect athree-phase rotating field with the flux m in the airgap between thetooth 46 and rotor 18 leading the flux es; in the airgap between thetooth 45 and rotor 18 which latter flux in turn leads the flux Sz in theairgap be tween the tooth 44 and rotor l8. It will be understood thatthe same analysis applies to the pole 2| except that the fluxes difierin phase with respect to the fluxes in the pole 28 by 180 degrees.

The integral magnetic bridges 20a and 21a referred to above are, as inmy prior copending application, each provided with a suitablerestriction such as an aperture or as illustrated the notches 48 whichmay be designed of such a size as to control the concentration of fluxthrough the integral magnetic bridges 28a and 2Ia respectively. Byproperly adjusting ar calibrating the concentration of flux which may beaccomplished by varying the size of the grooves or 110501185 48, theseintegral magnetic bridges 28a and 2h: may be worked at high flux valuesthereby throwing the desired volume of flux through the airgap betweenthe stator and rotor for the purpose of increasing the inductance of theshading coils as was mentioned above as desirable to cause the shadingcoils to have their maximum effect. However, as can be noted in Fig. 2,the width of the notches 48 is small compared to either the adjacentstator teeth 41 or 44 so that the pole-face, pole-arc ratio is notsubstantially reduced by the notch. From Fig. 2 it is to be noted thatthe notches 48 separate the adjacent stator teeth 44 and 41 of adjacentpole-faces. It is apparent that the flux passing through each of theintegral magnetic bridges 28a and 21a detracts from the aircutawayportion of the stator similar to a static transformer.

gap i'lux which latter is the only torque producing flux. Consequently,the efl'ective size of the magnetic bridges controlled by the size ofthe notches 4! is rather critical. It is desirable to maximize theproduct of the airgap flux and the sine oi the angle of phase lagbetween unshaded and shaded fiux; It is, of course, well understood thatboth the magnitude and the phase of the flux in an induction motordetermine the torque produced. The airgap flux is increased byincreasing the size of the notches 48 while the sine of the angle isincreased by decreasing the notches 48. It should be noted further thatthese notches 48 are oil-center and in fact, as indicated in Fig. 2 ofthe drawings, are both positioned to the left of the center line isthereby providing an additional non-symmetrical relationship as is fullydescribed and claimed in my copending application referred to above. I

In accordance with the present invention, a four-phase, two-pole coretype motor is provided by providing in each pole 20 and 2! a reluctancegap 49 defined with reference to the pole 20 for example between thetooth 41 and the rotor l8. Preferably, these reluctance gaps 49 comprisea laminations so as to define over a small area between the points 46aand 41a of the pole 20, and similarly with reference to the pole 2|, asubstantially increased airgap between rotor l8 and the stator Ill. Thecut away portion defining the reluctance gap 49 has a circumferentialarea of the order of one-half the circumferential area of the main poletooth 46 and the two cutaway portions defining the reluctance gaps 49 ineach pole are somewhat nonsymmetrically disposed relative to the centerof being opposite of pole 20 with the point 52 of the pole 2| while theline interconnects the point 41a of the pole with the point 53 of thepole 2!. The flux in the reluctance gaps 49 defined between the rotor 18and the teeth 41 as was mentioned above is designated by the symbol 111'in Fig. 2 of the drawings and leads the main flux m by a predeterminedamount so that effectively there is provided a four-phase motor with thefluxes in each pole being divided into four groups displaced in phasefrom each other with the flux (fir leading the main flux pm, and thefluxes s1 and s2 lagging the main flux by different varying amounts. Thereluctance gaps for the specific motor referred to above are about milsgreater than the airgap between the rotor and the other portions of thepoles 20 and 2|.

The reason why the flux 4- in the reluctance gap 49 leads the main fluxem in the shaded-pole induction motor of the present invention isperhaps most readily'apparent from the following discussion. It has beenstated that from one point of view an induction motor may be regarded asdirectly evolved from a direct current shunt motor; from another pointof view, that it is closely akin to the static transformer; from stillanother that it resembles a generator feeding a fictitious resistance;and that all of these points of view are useful in understanding thephysicalbehavior of such an induction motor. Since the phenomenainvolved in the shaded-pole motor of the present invention is equallyapplicable to both locked rotor and running conditions thereof, one canconsider the locked rotor condition as being It is apparent that thecircuit defined by any particular pair of rotor bars 22 andthe-connecting end plates act like a shading ring or shading coil to 151magnetic fiux linked with this circuit. Furthe more, the induced currentin this circuit due the pulsating magnetic field produced by the iielstructure will lag behind the electromotive for! induced in the rotorbar electric circuit undc consideration by an amount depending upon thresistance of this rotor'bar circuit and its in ductance. Although theinductance of this rote the end plates 23, rent which flux passesthrough the airgap between the rotor and stator is equal to the maggap,since the remainder of the magnetic circuit is iron with negligiblereluctance relative to the reluctance of the airgap. Since the flux isinversely proportional to the reluctance, the greater the airgap thegreater the reluctance and the when (these rotor bars are opposite thetooth 41 adjacent the reluctance gap 49. As a result, the secondarycurrent induced in the rotor bar circuit will lag more when the smallerairgap is eifective than when the larger airgap is efiective and theairgap flux due to the combined action of the primary magnetomotivewhere the airgap is large. Consequently, the flux where the airgap islarge, since it lags the primary current by a lesser angle, wouldobviously lead the flux where the airgap is small which lags the primarycurrent by a greater angle. In other words, the flux (1): would lead theflux m.

It is obvious that if the notches or restrictions 48 were not present inthe magnetic bridges 20a flux entering the rotor magnetic restrictions.increased because of of the so-called reluctance gaps 59 in the statorcurve A represents the speed-torque curve of the motor disclosed andclaimed in my prior copending application which incidently shows asubstantial improvement over any other prior art shaded-pole motors. Thecurve B represents the speed-torque curve of the motor disclosed inFigs. 1 to 3 of the drawings which differs from the motor of my priorcopending application substantially only in the provision of thereluctance gaps 4!. It will be observed from Fig. 4 of the drawings thatthe starting torque has been increased by about 50 percent. It will benoted that the pull-out torque which is in the neighborhood of 2500 R.P. M. has been increased over 30 percent and in addition, thecharacteristic dip indicated at A1 in the curve A has disappeared and nodip in the speed torque curve B is present. In addition the efllciencyof the motor has furthermore been increased by one-third or more:

The reason why the marked increase in emciency and torque is obtained aswell as the elimination of the characteristic dip of the speedtorquecurve is not fully understood. Any logical argument or theory that couldbe advanced as to why the marked improvements in performance referred toabove are obtained falls down when it is considered that extensive testshave shown that the employment of a reluctance gap as described aboveeither made no improvement or actually caused inferior performance of atwo-pole core type shaded-pole motor employing only a single shadingcoil per pole. Furthermore, tests in connection with a four-pole motorsuch as is disclosed and claimed in my prior Letters Patent of theUnited States No. 2,071,224 which employs the integral magnetic bridgeas well as one shading coil per pole, clearly showed that no improvedperformance was obtained with the incorporation of a reluctance gap.Tests were also made on a motor such as is disclosed in Patent No.2,071,224 referred to above except that two shading coils per poleinstead of one were employed but no improved performance was found. Inother words, the incorporation of the reluctance gap of the presentinvention in a shaded-pole motor which in the case of the integralmagnetic bridge core type two-pole motor described above having aplurality of shading coils per pole provided such a marked improvementas described above had either no appreciable efiect or actually causedan inferior performance when incorporated in either a two-pole motorhaving a single shading coil per pole or when incorporated in afour-pole motor having either one or a plurality of shading coils perpole. The marked improvement of the present invention might be explainedon the basis that eccentricities of the leading magnetic field producedby the reluctance gap of the present invention combine with theeccentricities of the lagging magnetic fields produced by virtue ofemploying a plurality of shading coils on each pole in a manner tocompensate for these eccentricities to provide the very desirablecharacteristics obtained, namely, the absence of a dip in thespeedtorque curve and the greatly increased torque and efficiency. Itshould be understood that applicant does not intend to be bound by thetheory or explanation advanced above for the greatly increasedperformance of the shaded-pole motor of the present invention in theevent that it should later prove to be the incorrect theory in supportof the improved performance.

In view of the detailed description included above. the operation of theeflective four-phase,

0 shaded-pole motor of the present invention which is energized from asingle phase source will be understood by those skilled in the art. Thereluctance gap section, the main pole section and the sections of thepole embraced by one and more than one shading coil produce a fieldwhich tends to approach the well known rotating field to produce greatlyincreased starting and operat ing torques and increased efiiciency asdiscussed above and as is apparent from an examination of the curves ofFig. 4 of the drawings. As was mentioned above an odd number of rotorbars might be employed together with a symmetrical relation 7 betweenthe shading coils and rotor bars in which case it would be desirable forthe pole face width of the tooth 41 which is also the width of thereluctance gap 49 to be equal to one and one-half times the rotor barpitch in order to minimize the tendency of the rotor to lock at twopoints as would otherwise be the case when employing an odd number ofrotor bars.

Referring now to Fig. 5 of the drawings, there is illustrated analternative construction embodying the present invention in which therotor and magnetic circuit immediately surrounding the airgap includingthe shading coils is identical with that of Figs. 1 to 3 of thedrawings. Accordingly, the corresponding parts of Fig. 5 have beendesignated by the same reference numerals. The motor of Fig. '5 differsfrom the motor described above primarily in that it comprises, based onthe transformer analogy, a shell-type motor as distinguished from a coretype motor.

As illustrated, the shaded-pole motor of Fig. 5 also comprises atwo-part stator generally designated at 55 comprising two separate setsof laminations 56 and 51. The laminations 56 preferably stamped fromsheet material in the same manner as the laminations I l of Fig. 1 havea generally rectangular external configuration with the central part cutaway so as to define a substantially rectangular opening 68. Thelaminations 56 are held in stacked relationship by suitable rivets 58and effectively define a rectangular annulus or shell enclosing theother portions of the motor whereby the characteristic designation ofshell-type motor arises. The laminations 51 form a removable insertsimilar to the insert l2 in the motor of Fig. 1 of the drawings andthese laminations 51 are also stacked and held in stacked relationshipby suitable rivets 5!. The laminations 51 are provided with a circularopening 11, identical with the opening H in the stator ill of the motorof Fig. 1, for receiving the cylindrical rotor l8. The portions of thelaminations 51 surrounding the circular opening II are formed inidentically the same manner as the laminations ll of Figs. 1 and 2 ofthe drawings surrounding the opening I1 and no further descriptionthereof will be included except that all of the portions thereof aredesignated by the same reference numerals as in Figs. 1, 2, and 3, ofthe drawings. The laminations 51 which form the removable insert areeach provided at either end thereof with a notch or groove I whichcooperates with protuberances 6| formed on each lamination 56. With thisarrangement, suitable interlocking means are provided whereby thelaminations 51 in stacked form are held in rigid interlockedrelationship with the laminations 56 in stacked form in the same mannerthat the interlocking means 16 of Fig. 1 of the drawings holds theinsert I2 in position relative to the remainder of the stator l0.

Fig. of the drawings employs two identical windings 62 and 63 mounted onsuitable spools B4 and 65. respectively, which are positioned on theends of the insert defined by the laminations 51. One of the windings isshown in section better to illustrate the invention. If desired,suitable insulating spacers such as 66 may be provided to hold thespools 64 and 65 supporting the windings i2 and 63, respectively, infixed position. These insulating spacers may be formed of wood or anyother suitable material. Preferably strips of insulating material 61 areinterposed between the shell-type stator laminations l8 and the fieldwindings mounted on spools 84 and 65, respectively.

the motor of Figs. 1 to 3 described above, no further discussion isdeemed necessary. The shell-type motor provides a symmetricalrelationship between the rotor shaft 24 and the frame of the motor whichmay be advantageous in certain applications.

combination of a laminated stator core having a poles, a rotor of asquirrel cage type plurality of having equally spaced conducting rotorbars ad- Jacent to the cylindrical surface exposed to the faces of saidpoles, a plurality of shading rings associated with each pole, one ofsaid shading rings linking a first section of its associated pole,another of said shading rings associated with said pole linking a secondsection comprising all of said first section plus an additional sectionof said pole, the remainder of said pole comprising third and fourthsections, the airgaps between said rotor said first, second and thirdsections being substantially the same, and the airgap between saidfourth section and said rotor being substantially greater than saidfirst-mentioned airgaps, each of said sections having an extentsufficient to include at least two of said rotor bars, said foursections being so related relative to said rotor that a point on saidrotor during rotation thereof moves past first said fourth section andthen said third, second and first sections in that order, said coreincluding high reluctance portions joining said first section of eachpole with said fourth section of the adjacent pole, said reluctanceportions having an extent small compared with the spacing of said rotorbars, and a winding positioned on said core and adapted to be energizedfrom a single phase alternating current source.

2. An alternating current shaded-pole motor comprising a stack of statorlaminations defining a core having a circular opening therein, anenergizing winding adapted to produce an alternating current fiux insaid core with the two portions of said core on each side of apredetermined diameter of said opening defining a pair of field poleshaving opposed concave pole faces, a cylindrical squirrel cage rotordisposed in said opena high reluctance gap relative to the airgapbetween the rotor and the remainder of said first section so that a fluxleading the flux in said remainder of said first section enters therotor through said high reluctance gap, said high reluctance gap havingan extent sufficient to include at least two rotor bars, high reluctancebridges said bridges being very tips thereof interconnected by integralhigh reluctance magnetic bridges, a cylindrical squirrel cage rotorincluding spaced rotor bars positioned in said opening and rotatablerelative to said poles, said bridges having an extent which is smallcompared to the spacing of said rotor bars, and a plurality of shadingrings for each the rotor through said increased airgap, said portionextending about said opening for a distance sufllcient to include atleast two rotor bars said flux in said shaded section being divided intoa plurality of parts lagging the flux in said remainder of said firstsection by varying amounts, whereby each pole produces an eilectivemultiphase airgap flux and the efiiciency and starting torque of saidmotor are greatly increased.

4. In an alternating current shaded-pole motor of the integral bridgeshell type, a stack of stator laminations defining a core including aremovable laminated insert, means defining a substantially circularopening in said insert, a pair of energizing windings disposed on saidinsert one on either side of said opening adapted to produce analternating current flux in said core with the two portions of saidinsert on either side of said pair of field poles having opposed concavesurfaces, a cylindrical rotor positioned in said opening and rotatablerelative to said poles, a plurality of shading rings for each poledividing said concave surface of each pole into a first section fromwhich unshaded flux a plurality of parts lagging the flux in theremainder of said first section by varying amounts whereby an effectivemulti-phase flux is produced in each pole, said insert includingintegral high reluctance portions adjoining said poles, said highreluctance portions being very narrow so that said pole surfaces coversubstantially the entire cylindrical surface of said rotor.

5. In an induction motor of the shaded-pole type, the combinationcomprising a laminated stator having a plurality of poles, a rotor ofthe squirrel cage type having equally spaced conducting rotor barstherein, at least two shading rings associated with'each pole forproviding first and second pole sections in which the flux lags the fiuxin the remainder of said pole, one of said shading rings linking saidsecond pole section and all of said shading rings linking said firstpole section so that the flux in said first pole section lags a greateramount than said flux in said second pole section, the remainder of eachpole being divided into third and fourth sections, the air gaps betweensaidfirst, second and third sections and said rotor being substantiallythe same, the air gap between said fourth section and said rotor beinggreater than said first mentioned airgaps so that the flux in saidfourth section leads M the flux in said third section, said fourthsections extending along said rotor for a distance equal to at least oneand one-half times the spacing of said rotor bars, said sections beingso positioned relative to said rotor that during rotation thereof saidrotor bars move past said fourth section and then said third, second,and first sections in that order, said stator including integral highreluctance bridges joining said first section of each pole and saidfourth section of the adjacent pole, said high reluctance bridges havingan extent less than the extent of said fourth sections oi said poles.

EARLE W. BALL-ENTINE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Finch Mar. 14, 1944

